At a nuclear power plant, nuclear safety depends on the flawless and reliable operation of all the equipment and systems that prevent the occurrence of an accident or eliminate the consequences of failures, retain radioactive substances in designated areas and prevent them from being dispersed in the environment in the event of their release. The current level of safety of nuclear reactors guarantees that all systems are able to handle a failure on their own without endangering the population or the environment.
The fundamental guiding principle for the safety of a nuclear power plant is defence-in-depth. Defence-in-depth is based on 4 physical barriers and 5 levels of protection. Defence-in-depth is designed to compensate for potential human errors and equipment failure. It focuses on a series of protective barriers including barriers designed to prevent radioactive substances leaking into the environment. The principle also includes protection of the barriers themselves and other measures for protection of the population and environment against harm if the barriers are not entirely effective.
The reliability of every barrier is extremely high and the probability of all 4 barriers failing at once and allowing radioactive material to disperse into the environment is very low
The first barrier is the ceramic pellet holding fuel material in a grid structure.
The second barrier is the zirconium alloy that hermetically seals the pellets into fuel rods
The third barrier is formed by the hermetically sealed pressure system of the primary circuit, which includes the reactor pressure vessel and is able to prevent the cooling medium containing radioactive substances from leaking into the environment at all foreseen temperatures and pressures.
level: conservative design, quality assurance, control activities, safety culture;
level: operations management, including responses to abnormal (degraded) operating states to ensure the integrity of the first two physical barriers;
level: safety and protection systems performing the three fundamental safety functions;
level: measures to maintain the integrity of containment;
